This invention relates generally to acoustic and mechanical attenuating and focusing materials and devices, and more particularly to metamaterials suitable for mechanical vibration isolation of sensors, as well as acoustic shields from sound that propagate through a fluid, for example air or underwater. This invention relates as well to mesoscale, compact devices for focusing, coupling, localization and controlling the propagation in general of elastic and acoustic waves utilizing sub-wavelength building blocks.
Phononic metamaterials enable the manipulation of both elastic and acoustic waves in different media, from attenuation (including absorption and reflection) to coupling, tunneling, negative refraction and focusing. In particular, the attenuation of vibrations, such as vector mechanical vibrations through a solid, or a scalar acoustic vibration in a fluid, such as in air or water, is important technologically for applications where the presence of such vibrations affects the intended performance of the device or entity in question, such as, but not limited to, a sensor or a source of emission, such as a laser which may suffer from reduction in performance due to losses arising from the coupling of mechanical modes into an acoustic or an elastic medium. Another example of this is the attenuation of high frequency (>2 KHz) sound in acoustic hearing.
Conventional attenuating materials utilize thermally-coupled dissipation mechanisms to reduce the intensity of incident vibrations through dissipation-induced heating of the material. Such a dissipative mechanism does not have frequency selectivity; the attenuation performance is dependent monolithically on the thickness of the material being utilized, governed by the mass-density law, given by
      T    =                  4        ⁢                  γexp          ⁡                      (                                          ik                2                            ⁢              d                        )                                                            (                          1              +              γ                        )                    2                -                                            (                              1                -                γ                            )                        2                    ⁢                      exp            ⁡                          (                              2                ⁢                                  ik                  2                                ⁢                d                            )                                            ,where T is the amplitude transmission coefficient, d is the sample thickness, and γ is the impedance ratio between the different media on both sides of the interface (one medium typically being air). In general, at low frequencies,
      T    ≅                  i        ⁢                                  ⁢        2        ⁢                              ρ            ⁢                                                  ⁢            1            ⁢            K            ⁢                                                  ⁢            1                                      ω        ⁢                                  ⁢        p2d              ,hence the lower the frequency, the heavier the mass density needed to achieve the same amount of transmission (attenuation) or the thicker the material required. These materials hence suffer from inadequate low frequency attenuation and implementation issues. As there are characteristic frequency and intensity ranges that have been identified as being detrimental to performance of, as well as to cause mechanical damage to, the device, it is hence desirable to provide an isolating/attenuating material that can be designed to have high attenuation within a target frequency range for broadband vibration and acoustic isolation. Also for certain applications, it is highly desirable to have excellent transmission of certain frequency bands (e.g. receipt of signals) and the ability to design the transmission spectrum across a wide range of frequencies is highly advantageous.
A relevant area of application would be to reduce particular vibrations of a set of frequencies from a body or entity or device that needs to be mechanically attached to a platform, hence providing a tradeoff between the requirement for mechanical stability and vibration isolation.
An object of the invention is a structured metamaterial that possesses multiple high-frequency spectral gaps capable of providing acoustic and/or mechanical vibration attenuation at high frequency ranges while retaining mechanical stability with a larger structure and permitting excellent transmission in selected regions.
Another object of the invention relates to acoustic and elastic metamaterials designated as sub-wavelength, meaning that they are able to control waves with wavelengths much greater than the physical structure, such as the unit cell dimension. These metamaterials are also capable of exhibiting double negative index behavior, leading to a general wave phenomenon known as negative refraction. These devices however, typically require the incorporation of multiple materials (usually greater than 2), to obtain the required double negative index behavior. This presents inherent issues related to scaling down the intended device application to smaller scales, due to fabrication challenges involved in incorporating multiple materials. Some of the issues include the requirement of introducing and fabricating the different material components and adhering the different material interfaces together, which due to their different mechanical properties, limits the choice of material components. This presents challenges in applications involving functioning under dynamic variation in the material mechanical properties, such as thermal fluctuations, thermal cycling and thermal stress arising from differences in thermal expansion coefficients. One further complication of requiring multiple material components include the immediate reduction of throughput due to the fabrication requirements, as well as more complexity in manufacturing, leading to greatly reduced yields.
This invention describes a structured metamaterial that may be fabricated out of a single material that possesses such sub-wavelength negative index behavior, hence addressing several of the above challenges.